Lanthanide Hexacyanidoruthenate Frameworks for Multicolor to White-Light Emission Realized by the Combination of d-d, d-f, and f-f Electronic Transitions

We report an effective strategy toward tunable room-temperature multicolor to white-light emission realized by mixing three different lanthanide ions (Sm3+, Tb3+, and Ce3+) in three-dimensional (3D) coordination frameworks based on hexacyanidoruthenate(II) metalloligands. Mono-lanthanide compounds, K{LnIII(H2O)n[RuII(CN)6]}·mH2O (1, Ln = La, n = 3, m = 1.2; 2, Ln = Ce, n = 3, m = 1.3; 3, Ln = Sm, n = 2, m = 2.4; 4, Ln = Tb, n = 2, m = 2.4) are 3D cyanido-bridged networks based on the Ln–NC–Ru linkages, with cavities occupied by K+ ions and water molecules. They crystallize differently for larger (1, 2) and smaller (3, 4) lanthanides, in the hexagonal P63/m or the orthorhombic Cmcm space groups, respectively. All exhibit luminescence under the UV excitation, including weak blue emission in 1 due to the d-d 3T1g → 1A1g electronic transition of RuII, as well as much stronger blue emission in 2 related to the d-f 2D3/2 → 2F5/2,7/2 transitions of CeIII, red emission in 3 due to the f-f 4G5/2 → 6H5/2,7/2,9/2,11/2 transitions of SmIII, and green emission in 4 related to the f-f 5D4 → 7F6,5,4,3 transitions of TbIII. The lanthanide emissions, especially those of SmIII, take advantage of the RuII-to-LnIII energy transfer. The CeIII and TbIII emissions are also supported by the excitation of the d-f electronic states. Exploring emission features of the LnIII–RuII networks, two series of heterobi-lanthanide systems, K{SmxCe1–x(H2O)n[Ru(CN)6]}·mH2O (x = 0.47, 0.88, 0.88, 0.99, 0.998; 5–9) and K{TbxCe1–x(H2O)n[Ru(CN)6]}·mH2O (x = 0.56, 0.65, 0.93, 0.99, 0.997; 10–14) were prepared. They exhibit the composition- and excitation-dependent tuning of emission from blue to red and blue to green, respectively. Finally, the heterotri-lanthanide system of the K{Sm0.4Tb0.599Ce0.001(H2O)2[Ru(CN)6]}·2.5H2O (15) composition shows the rich emission spectrum consisting of the peaks related to CeIII, TbIII, and SmIII centers, which gives the emission color tuning from blue to orange and white-light emission of the CIE 1931 xy parameters of 0.325, 0.333.

S5 Figure S4. Solid-state room-temperature UV-vis-NIR absorption spectra of 1-4 in the 220-1000 nm range, together with the enlargement of the 220-400 nm region presented in the inset. The spectra were normalized to the intensity at 220 nm.
S6 Table S1. Details of crystal data and structure refinement for 1-4.
S26 Table S8. Results of the SEM EDXMA microanalysis of the lanthanide ions' compositions, expressed as the Ce/Tb ratio, in compounds 10-14. The representative measurements points/areas 1-3 are shown in Figure S18. It is important to note that for compound 14 a few measurement points revealed the Ce/Tb ratio below the reliable detection limit of 0.001. The peaks related to the Ce atoms were observed but they were too small be reliably    with the experimental P-XRD patterns of 2 (top, a hexagonal phase, Table S1, Figure 1) as well as 3 and 4 (bottom, orthorhombic phases, Table S1, Figure 1). Figure S23), adopting the P63/m (compound 2) and Cmcm (compounds 15, 3, and 4) space groups and using the LeBail fitting procedure performed with an EXPO 2014 software. S7 (7) 7.4840 (7) 14.157 (2) 686.69 (14) 15 7.456(2) 12.839 (5) 14.317 (6) 1370.5(9)  Table S10. Table S10. Results of the SEM EDXMA microanalysis of the lanthanide ions' composition, expressed as the Ce/Sm and Ce/Tb ratios, in 15. Note that only five measurements points are here presented with the reliable values while ten measurements points were shown for compounds 5-14 (Tables S6 and S8, Figure S24). The other five points were measured for 15 but they showed the Ce/Sm and Ce/Tb ratios smaller than 0.001 which was below the reliable detection limit of the used apparatus. However, they represent the dispersion of the Ce atoms within the crystals;

3, and 4 (
thus, these values were used for the calculation of the average ratios (*). These very small values of Ce/Sm and Ce/Tb correspond to some measurements points while all measurement areas (a few selected areas instead of single points) always showed the higher Ce/Sm and Ce/Tb ratios; thus, these smaller values are related to the non-ideally homogenous dispersion of the Ce atoms within the different fragments of the crystals. (2-dehyd) and its rehydrated form (2-rehyd) (a), emission spectra for 15 and its thermally dehydrated form (15dehyd), both for the indicated excitation of 317 nm, (b), and emission spectra for 15 and its thermally dehydrated form (15-dehyd), both for the indicated excitation of 295 nm (c). Note that all spectra were normalized to the selected peaks occurring for both hydrated as well as dehydrated phases. The real emission intensity after dehydration was much weaker than for the as-synthesized sample for the case of compound 15 while the dehydration only weakly affects the intensity for the case of compound 2.  S40 Figure S29. Alternate-current (ac) magnetic characteristics of 2, including the frequency dependences of the in-phase magnetic susceptibility, χM′(ν), the out-of-phase magnetic susceptibility, χM″(ν), and the related Argand plot, χM″(χM′), under the variable temperature from the 1.8-3.0 K range at Hdc = 1 kOe. The lines are only to guide the eye.

Discussion on magnetic properties of 2-4 (Comment to Figures S27-S29)
The direct-current (dc) magnetic characteristics of 2-4 are presented in Figure S27. The room temperature value of the magnetic susceptibility-temperature product, χMT for 2 is 0.73 cm 3 mol −1 K, which is very close to the value of 0.80 cm 3 mol −1 K expected for the isolated free Ce 3+ ion with the 2 F5/2 ground multiplet. S8,S9 Upon cooling, the χMT decreases slowly down to 0.66 cm 3 mol −1 K at 20 K. Below 20 K, there is a more significant decrease of the χMT down to 0.61 cm 3 mol −1 K −1 at 1.8 K. Molar magnetization of 2 at 1.8 K shows the monotonous increase upon the increasing magnetic field reaching 1.2 μB at 70 kOe. These magnetic characteristics can be mainly ascribed to the single-ion properties of the Ce III complexes embedded in 2 which is the gradual cooling-induced depopulation of the excited mJ levels within the 2 F5/2 ground multiplet. Due to the separation of the Ce III centers by diamagnetic hexacyanidoruthenate(II) metalloligands, the inter-lanthanide magnetic interactions are canceled, and their minor role can appear only at the lowest temperatures below 10 K.
For 3, the χMT reaches 0.48 cm 3 mol −1 K at 212 K (the highest accessible temperature of the reliable measurement due to the overall very weak magnetic signal) and decreases nearly linearly upon cooling down to the very low value of 0.03 cm 3 mol −1 K at 1.8 K. The room-temperature value is much higher than the 0.09 cm 3 mol −1 K expected for the isolated Sm 3+ ion with the 6 H5/2 ground multiplet; however, this is a typical case as the Sm 3+ ions reveal the closely lying excited 6 HJ multiplets that contribute to the overall magnetic signal at high temperatures. S8,S9 At 1.8 K, the molar magnetization of 3 monotonously increases reaching 0.13 μB at 70 kOe without saturation which is typical for isolated lanthanide(III) centers. The observed characteristics can be reasonably explained by the single-ion properties of the Sm III complexes, that is the cooling-induced depopulation of the excited 6 HJ multiplets and further higher-lying mJ levels within the ground multiplet. No sign of inter-lanthanide magnetic interactions is observed, as expected for the Sm III centers with the very low magnetic moment, additionally being separated in the framework by the diamagnetic cyanido complexes.
At room temperature, the χMT for 4 is 12.1 cm 3 mol −1 K, which is very close to the value of 11.8 cm 3 mol −1 K, expected for the isolated free Tb 3+ ion with the 7 F6 ground-state multiplet. S8,S9 This value only weakly decreases upon cooling down to 50 K, while the abrupt decrease of χMT value is observed, down to 4.26 cm 3 mol −1 K at 1.8 K. The fielddependent magnetization curve shows the monotonous increase up to the value of 5.2 μB at 70 kOe. Similar to 2 and 3, these dc magnetic characteristics can be mainly assigned to the single-ion properties of the Tb III complexes present in the structure of 4; however, the relatively large drop of the magnetic signal at the lowest temperatures can be partially assigned to the not fully suppressed magnetic interactions between lanthanide ions through the diamagnetic hexacyanidoruthenate(II) linkers.
As 2-4 contain the lanthanide(III) complexes that can reveal the substantial magnetic anisotropy leading to the Single-Molecule Magnet (SMM) behavior, S9 we tested the related alternate-current (ac) magnetic properties. Compounds 3 and 4 do not exhibit the noticeable signal of the out-of-phase magnetic susceptibility, χ″M in the accessible frequency range of 1-1000 Hz at 1.8 K which indicates the lack of slow magnetic relaxation effects that are characteristic of the SMMs. This indicates the poor magnetic anisotropy of the related Sm III and Tb III complexes, respectively. The onset of slow magnetic relaxation is observed in the Ce III -containing 2 through the non-negligible χ″M signal in the highest frequency range under the applied dc field at 1.8 K ( Figure S28). The temperature-variable ac magnetic data for the optimal dc field of 1000 Oe shows the shift of the χ″M signal toward higher frequencies upon heating leading to the S42 complete disappearance of the signal in the accessible frequency range above 2.6 K ( Figure S29). This indicates nonnegligible but rather weak magnetic anisotropy of the Ce III complexes. No maxima on the χ″M(ν) are detected in the accessible frequency range which precludes the more detailed analysis. There is also a second, slower relaxation process that occurs in 2 for the high dc magnetic fields ( Figure S28); however, it does not shift on the frequency scale upon the increased dc field or temperature suggesting that it is most probably due to the magnetic dipolar interactions, not related to the single-ion properties of the Ce III centers. S10 These results show that the investigated lanthanide complexes in 2, 3, and 4, exhibit very weak magnetic anisotropy. This can be correlated with the respective coordination environments (Figure 1) involving cyanido and aqua ligands within the complexes of high coordination numbers leading to rather long metal-ligand distances. Therefore, there is no clear source of the modulation of the 4fmetal ion electron density (there is a lack of, e.g., strongly coordinating negatively charged ligands) that could provide the distinct SMM effect. S9,S11